1. Field of the Invention
The present invention relates to the field of the treatment of water with a view to its purification by biological means, notably discharge water and waste water or any water containing phosphorus in various forms and, possibly, ammonia nitrogen. It relates quite especially to a method designed to eliminate carbon, nitrogen and traces of phosphorus by biological dephosphatization on biofilters.
2. Description of the Prior Art
It is known that waste water can be treated by the biological method known as the activated sludge method wherein a biomass is kept in suspension in the presence of oxygen and carbon-containing nutriment. The bacteria present in the biomass degrade the polluted matter and are separated from the purified water in a clarifier. It is also possible to maintain autotrophic bacteria in the system, enabling the conversion of nitrogen ammonia into nitric nitrogen if a great sludge age of at least six days is adopted in the system. If a period of non-aerated (or anoxic) contact is incorporated into the system, it is possible to force the heterotrophic bacteria to degrade the nitrates in the presence of carbon-containing nutriment. If, moreover, the bacteria are made to undergo a systematic alternation of the anaerobic and aerobic states, an excess accumulation of phosphorus compounds can be caused in the bacteria.
A method such as this, with the above-described principles, has been known since 1974 under the process name of "BARDENPHO".
All the configurations currently available for the total elimination of nitrogen and phosphorus by biological means constitute variants of the above-mentioned standard activated sludge method. A variety of possible arrangements of the anaerobic, anoxic and aerobic zones has led to the development of several methods in the course of the past 20 years, notably those known by the names of Phostrip, A/0, Phoredox, UCT, Biodenipho, etc.
These configurations share the common feature of having an anaerobic basin at the front of the hydraulic flow designed to expose the biomass to an alternation of aerobic/anaerobic conditions. To reduce the nitrates in the anaerobic zone, several internal circuits for the recirculation of the sludges between anoxic and anaerobic zones are used in order to obtain efficient performance in terms of biological dephosphatization.
In these methods, several factors restrict the efficiency of the dephosphatization: these are notably the exposure time of the biomass to the anaerobic conditions which, most frequently, has to be limited to between 0.5 and 2 hours, and the value of load per mass which remain low because of the constraints on the age of the sludges dictated by the nitrification. This restricts the application of the methods in question to the treatment of effluents, for which the COD (chemical oxygen demand)/total P (phosphorus) ratio is high, notably higher than 20. It is difficult, in these methods, to obtain very low residual rates of phosphorus owing to the impossibility of increasing the load of carbon-containing nutriment for heterotrophic bacteria and owing to the fact that the subsequent clarifier releases suspended solids that are highly rich in phosphorus. Thus, it is necessary to add an additional physical/chemical process to the treatment of waste water to achieve a very low residual rate of phosphorus.
More recently, other methods for the purification of waste water have been proposed in which it is no longer activated sludge systems with freely circulating bacteria that are used but so-called "biofilter" reactors are used wherein the bacteria are fixed to a support. Among techniques of this kind, we may cite the processes described in the published French patents Nos. 2 604 990 and 2 632 947. These methods lead to excellent results in the elimination of carbon and nitrogen but, owing to the fixing of bacteria, it cannot provide for a satisfactory biological dephosphatization.
It has now been found that it is possible to resolve this problem of the high elimination of phosphorus in the operational context of biofilter methods by a technique which can be used to obtain a maximum rate of carbon-containing substrate in an anaerobic cell so as to achieve an absence of electron (O.sub.2 and NO.sub.3 ) acceptors during the non-aerated phase and minimize the losses of suspended solids (bacteria).